At a yearly production rate of over 40 million tons, polyethylene remains one of the most valuable synthetic polymers in the world. It has found application in products ranging from grocery bags and milk containers to high performance fibers and medical devices. Its versatility stems from our ability to tune the material's crystallinity, mechanical strength, and thermal stability by altering the architecture of the individual polymer chains. However, the rising number of applications for polyethylene demands its material properties to be broadened even further.
Most efforts directed to altering the physical properties of polyethylene and other polymers have focused on methods for increasing the structural complexity of the polymer rather than on making more simple topological modifications that could be quite significant. For example, cyclization of a linear precursor to form a cyclic polymer conceptually varies the structure only minimally, but the physical properties of a macrocycle would be expected to differ markedly from the linear counterpart as a result of the restriction on conformational freedom and overall dimensions. For example, cyclic polymers are less viscous, exhibit higher glass transition temperatures, and have smaller hydrodynamic volumes and radii (Rg) than their respective linear analogues. See Semlyen, Cyclic Polymers (Kluwer Academic, Dordrecht, The Netherlands, ed. 2, 2000).
Although cyclic polymers have been previously synthesized, access to high molecular weight material (MW>100 kDa), which is often required for many polymers to show their characteristic physical properties, has been extraordinarily difficult. Ibid. The typical synthetic route involves preparation of linear polymeric precursors that contain reactive end groups, followed by intramolecular coupling under highly dilute conditions. Alternatively, the balance between linear and cyclic products that occurs with many types of polymerization reactions (e.g. polycondensations, metathesis polymerizations, etc.) may be shifted to maximize formation of cyclic product (which again generally involves using low concentrations). Incomplete cyclizations or undesired side reactions are common for both approaches and therefore elaborate purification procedures are often required to remove the acyclic contaminants. See Lee et al. (2002) Macromolecules 35:529. Furthermore, many monomers, including ethylene, are not amenable to these types of polymerizations. As a result, there are very few reported examples of cyclic polyethylenes, especially in the high molecular weight (MW>104 Da) regime. See Höcker et al. (1977) Makromol. Chem. 178:3101 and Shea et al. (1998) J. Org. Chem. 63:5746.
Accordingly, there is a need in the art for an improved technique to prepare macrocyclic polymers. An ideal process would not involve linear intermediates, but proceed by way of a growing cyclic structure into which cyclic olefin monomers are successively inserted. In addition, an ideal method for synthesizing macrocyclic polymers would result in an easily isolable and stable structure without incorporation of undesired substituents or functional groups. The present invention is directed to the aforementioned need in the art, and makes use of Group 8 transition metal alkylidene complexes as polymerization catalysts of such a polymerization reaction.
Transition metal alkylidene complexes, particularly ruthenium and osmium complexes, have been described in U.S. Pat. Nos. 5,312,940, 5,342,909, 5,831,108, 5,969,170, 6,111,121, and 6,211,391 to Grubbs et al., assigned to the California Institute of Technology. The complexes disclosed in these patents all possess metal centers that are formally in the +2 oxidation state, have an electron count of 16, and are penta-coordinated. These catalysts are of the general formula XX′M(LL′)=CRR′ wherein M is a Group 8 transition metal such as ruthenium or osmium, X and X′ are anionic ligands, L and L′ are neutral electron donors, and R and R′ are specific substituents, e.g., one may be H and the other may be a substituted or unsubstituted hydrocarbyl group such as phenyl or C═C(CH3)2. Such metathesis catalysts include those that have been prepared with phosphine ligands, e.g., triphenylphosphine or dimethylphenylphospine, exemplified by phenylmethylene-bis(tricyclohexylphosphine)ruthenium dichloride wherein “Cy” is cyclohexyl. See U.S. Pat. No. 5,917,071 to Grubbs et al. and Trnka and Grubbs (2001), cited supra. These compounds are highly reactive catalysts useful for catalyzing a variety of olefin metathesis reactions.
More recently, significant interest has focused on such transition metal alkylidene catalysts wherein one of the phosphine ligands is replaced with an N-heterocyclic carbene ligand. See, e.g., Trnka and Grubbs, supra; Bourissou et al. (2000) Chem. Rev. 100:39-91; Scholl et al. (1999) Tetrahedron Lett. 40:2247-2250; Scholl et al. (1999) Organic Lett. 1(6):953-956; and Huang et al. (1999) J. Am. Chem. Soc. 121:2674-2678. N-heterocyclic carbene ligands offer many advantages, including readily tunable steric bulk, vastly increased electron donor character, compatibility with a variety of metal species, and improved thermal stability. See Scholl et al. (1999) Tetrahedron Lett. 40:2247-2250; Scholl et al. (1999) Org. Lett. 1:953-956; Chatterjee et al. (2000) J. Am. Chem. Soc. 122:3783-3784; and Bielawski et al. (2000) Angew. Chem. Int. Ed. 39:2903-2906, A representative of these second generation catalysts is the ruthenium complex (IMesH2)(PCy3)(Cl)2Ru═CHPh wherein Cy is cyclohexyl, Ph is phenyl, and Mes represents mesityl (2,4,6-trimethylphenyl).
As noted above, these complexes have been used to catalyze a variety of olefin metathesis reactions, including polymerization reactions. To date, however, there has been no disclosure of a method for efficiently synthesizing high molecular weight, stable, and readily isolable cyclic polymers. The invention now provides a method for synthesizing such cyclic polymers using a cyclic analog of the transition metal alkylidene complexes described above as a polymerization catalyst.